Wellbores are typically completed with a cemented casing across the formation of interest to assure borehole integrity and allow selective injection into and/or production of fluids from specific intervals within the formation. It is necessary to perforate this casing across the interval(s) of interest to permit the ingress or egress of fluids. Several methods are applied to perforate the casing, including mechanical cutting, hydro-jetting, bullet guns and shaped charges. The preferred solution in most cases is shaped charge perforation because a large number of holes can be created simultaneously, at relatively low cost. Furthermore, the depth of penetration into the formation is sufficient to bypass near-wellbore permeability reduction caused by the invasion of incompatible fluids during drilling and completion.
FIG. 1 illustrates a perforating gun 10 consisting of a cylindrical charge carrier 14 with explosive charges 16 (also known as perforators) lowered into the well by means of a cable, wireline, coil tubing or assembly of jointed pipes 20. Any technique known in the art may be used to deploy the carrier 14 into the well casing. At the well site, the explosive charges 16 are placed into the charge carrier 14, and the charge carrier 14 is then lowered into the oil and gas well casing to the depth of a hydrocarbon bearing formation 12. The explosive charges 16 fire outward from the charge carrier 14 and puncture holes in the wall of the casing and the hydrocarbon bearing formation 12. As the charge jet penetrates the rock formation 12 it decelerates until eventually the jet tip velocity falls below the critical velocity required for it to continue penetrating. As best depicted in FIG. 2A, the tunnels created in the rock formation 12 are relatively narrow. Particulate debris 22 created during perforation leads to plugged tunnel tips 18 that obstruct the production of oil and gas from the well.
Perforation using shaped explosive charges is inevitably a violent event, resulting in plastic deformation 28 of the penetrated rock, grain fracturing, and the compaction 26 of particulate debris (casing material, cement, rock fragments, shaped charge fragments) into the pore throats of rock surrounding the tunnel (as best shown in FIG. 2B). Thus, while perforating guns do enable fluid production from hydrocarbon bearing formations, the effectiveness of traditional perforating guns is limited by the fact that the firing of a perforating gun leaves debris 22 inside the perforation tunnel and the wall of the tunnel. Moreover, the compaction of particulate debris into the surrounding pore throats results in a zone 26 of reduced permeability (disturbed rock) around the perforation tunnel commonly known as the “crushed zone.” The crushed zone 26, though only typically about one quarter inch thick around the tunnel, detrimentally affects the inflow and/or outflow potential of the tunnel (commonly known as a “skin” effect.) Plastic deformation 28 of the rock also results in a semi-permanent zone of increased stress around the tunnel, known as a “stress cage”, which further impairs fracture initiation from the tunnel. The compacted mass of debris left at the tip 18 of the tunnel is typically very hard and almost impermeable, reducing the inflow and/or outflow potential of the tunnel and the effective tunnel depth (also known as clear tunnel depth).
The geometry of a tunnel will also determine its effectiveness. The distance the tunnel extends into the surrounding formation, commonly referred to as total penetration, is a function of the explosive weight of the shaped charge; the size, weight, and grade of the casing; the prevailing formation strength; and the effective stress acting on the formation at the time of perforating. Effective penetration is some fraction of the total penetration that contributes to the inflow or outflow of fluids. This is determined by the amount of compacted debris left in the tunnel after the perforating event is completed. The effective penetration may vary significantly from perforation to perforation. Currently, there is no means of measuring it in the borehole. Darcy's law relates fluid flow through a porous medium to permeability and other variables, and is represented by the equation seen below:
  q  =            2      ⁢                          ⁢      π      ⁢                          ⁢              kh        ⁡                  (                                    p              e                        -                          p              w                                )                            μ      ⁡              [                              ln            ⁡                          (                                                r                  e                                                  r                  w                                            )                                +          S                ]            Where: q=flowrate, k=permeability, h=reservoir height, Pe=pressure at the reservoir boundary, pw=pressure at the wellbore, μ=fluid viscosity, re=radius of the reservoir boundary, rw=radius of the wellbore, and S=skin factor.
The effective penetration determines the effective wellbore radius, rw, an important term in the Darcy equation for radial inflow. This becomes even more significant when near-wellbore formation damage has occurred during the drilling and completion process, for example, resulting from mud filtrate invasion. If the effective penetration is less than the depth of the invasion, fluid flow can be seriously impaired.
Inadequately cleaned tunnels limit the area through which produced or injected fluids can flow, causing increased pressure drop and erosion; increase the risk that fines migrate towards the limited inflow point and/or condensate banking (in the case of gas) occurs around the inflow point, resulting in significant loss of productivity; and impair fracture initiation and propagation.
Currently, common procedures to clear debris from tunnels rely on flow induced by a relatively large pressure differential between the formation and the wellbore. Perforating underbalanced involves creating the opening through the casing under conditions in which the hydrostatic pressure inside the casing is less than the reservoir pressure. Underbalanced perforating has the tendency to allow the reservoir fluid to flow into the wellbore. Conversely, perforating overbalanced involves creating the opening through the casing under conditions in which the hydrostatic pressure inside the casing is greater than the reservoir pressure. Overbalanced perforating has the tendency to allow the wellbore fluid to flow into the reservoir formation. It is generally preferable to perform underbalanced perforating as the influx of reservoir fluid into the wellbore tends to clean up the perforation tunnels and increase the depth of the clear tunnel of the perforation.
Underbalancing techniques maintain a pressure gradient from the formation toward the wellbore, inducing tensile failure of the damaged rock around the tunnel and a surge of flow to transport debris from the perforation tunnel into the wellbore. In other words, in conventional underbalance perforating, the wellbore pressure is kept below reservoir pressure before firing or detonating a perforation gun to create a static underbalance. FIG. 3 depicts the cleaning surge flow in an underbalanced system after explosive charges 16 are fired. After perforation, fluid flows from the formation through the tunnels. As the fluid flows through the tunnels and egresses through the tunnel openings 24, it takes with it the debris 22 formed as a result of perforation. Little, if any, debris 22 remains in the tunnels if a sufficient surge flow can be induced. However, underbalance perforating may not always be effective and/or may at times be expensive or unsafe to implement. Although underbalanced perforating techniques are relatively successful in homogenous formations of moderate to high natural permeability, in a number of situations, it is undesirable, difficult or even impossible to create a sufficient pressure gradient between the formation and the wellbore. For example, when the reservoir is shallow or depleted, the hydrostatic pressure of even a very light fluid or gas within the wellbore will result in only a very minimal underbalance being generated, which may be too low to induce a flow rate sufficient to clean the tunnel. Further, when working with a wellbore having open perforation tunnels, fluids will flow from the existing perforations as soon as a pressure difference is created, limiting the amount of underbalance that can be applied without adversely affecting tools in the wellbore or surface equipment. If perforation is performed without underbalance using conventional shaped charges, the fraction of unobstructed tunnels as a percentage of total holes perforated (also known as “perforation efficiency”) may be 10% or less.
Consequently, there is a need for an improved method of perforating a cased wellbore in situations where underbalancing techniques are undesired or unavailable. There is also a need for achieving superior inflow and/or outflow performance compared to that achieved with conventional shaped charges under the same perforating conditions.